An organic EL element is a light-emitting element that makes use of an electroluminescent phenomenon of an organic material. A typical organic EL element has a thin film transistor (TFT) substrate and an organic EL unit disposed on the TFT substrate. The organic EL unit has an organic light-emitting layer disposed between a lower electrode and an upper electrode. The TFT substrate has a TFT that is electrically connected to the lower electrode and that serves as a driving transistor controlling light emission from the organic EL unit (for example, Patent Literature 1).
In recent years, a transparent amorphous oxide semiconductor (TAOS) is used for TFT channel layers of organic EL elements so as to enable the organic EL elements to drive at high speed (A TFT having a channel layer made of TAOS is hereinafter referred to as “TAOS-TFT”).